text: The outer portion of a SLMD consists of a section of sealed, flat, semi-permeable polyethylene tubing. Sealed inside this tubing is a 1:1 mixture of a hydrophobic metal complexing agent and a long chain organic acid. The organic acid diffuses through the tubing to the outer surface, where the carboxylic acid portion can form stable complexes with calcium and magnesium ions in the water. This allows a waxy layer to slowly accumulate on the outside of the tube. the metal complexing agent continuously mobilizes into this waxy layer, where it can sequester metal ions from the water. The hydrophobic metal complexing agent most commonly used in SLMDs is an alkylated 8-hydroxyquinoline. Oleic acid is commonly used as the other half of the 1:1 hydrophobic reagent mixture, as it readily forms calcium oleates in the aqueous sampling media. In addition to the base device, hydrophobic plastic sheaths are sometimes used to house SLMDs in the field. Variable water flow can alter the sampling rates of metals by SLMDs, making a time-averaged concentration difficult to determine. By allowing liable metal analytes to diffuse to the SLMD's surface while limiting the diffusion of particulate, colloidal, or humic substances, these hydrophobic sheaths help reduce variability of SLMD uptake in faster moving waters.
  After being deployed for a known time interval, SLMDs can be recovered from the field for analysis. Washing with 20% nitric acid allows for the extraction of accumulated metals, and by using analytical techniques like inductively coupled plasma mass spectroscopy (ICP-MS) or atomic absorption spectroscopy (flame AAS)  to measure the concentration of metal in the extract, the amount of metal accumulated by the SLMD can be determined.